Conventionally, pneumatic rubber vehicle tires are produced by molding and curing a green (uncured) partially shaped tire in a molding press. The green tire is pressed outwardly against a mold surface (sometimes called a negative mold surface) by means of an inner fluid-expandable bladder. By this method the green tire is shaped against the outer mold surface which defines the tire tread pattern and configuration of the sidewalls. By application of heat and pressure the tire is molded and cured at elevated temperatures.
In general practice, the expansion of the bladder is accomplished by application of internal pressure to the inner bladder cavity which is provided by a fluid such as a gas, hot water and/or steam which also may participate in the transfer of heat for the curing or vulcanization of the tire. The tire after molding and curing is allowed to cool somewhat in the mold, sometimes aided by adding cold or cooler water supplied to the bladder. Then the mold is opened, the bladder is collapsed, including release of its internal fluid pressure, and the tire is removed from the tire mold. Such use of tire curing bladders is well known to those having skill in the art.
It is recognized that there is relative movement between the outer contacting surface of the bladder and the inner surface of the uncured tire during the expansion phase of the bladder. Likewise, there is relative movement between the outer contacting surface of the bladder and the cured inner surface of tire during the collapse of the bladder and the stripping from the cured tire after the tire has been molded and cured (vulcanized).
The bladder surface can tend to stick to a inner surface of the cured tire during the bladder collapsing part of the tire cure cycle. This adhesion may cause roughening of the bladder surface or of the inner surface of the tire. This reduces bladder durability and can produce defective tires. For this reason, it is conventional practice to precoat the inner surface of the uncured tires or the outer surface of the bladder with a lubricant in order to enhance lubricity between the outer bladder surface and inner tire surfaces during the entire molding operation. This lubricant has also been called a bladder lubricant, and is often a silicone polymer dispersed in a solvent or water.
It is to be appreciated that the release of the cured tire from its cure bladder in an industrial manufacturing setting is intimately associated with both the phenomenon of release (to prevent sticking or adhesion) and the phenomenon of lubrication (to enhance slipping) between the bladder and the adjacent tire surfaces.
Butyl rubber is commonly used in tire curing bladders. Butyl rubber is a copolymer of predominantly isobutylene with small amounts of diene monomers, usually isoprene, to give sufficient unsaturation to allow the butyl rubber to be crosslinked. Copolymers of isobutylene and paramethylstyrene which are subsequently brominated are being manufactured by the Exxon Chemical Company. These polymers are useful in many applications where butyl rubber is used. Oils such as castor oil, napthenic and other aromatic oils have been used in curing bladders to plasticize (make more easily deformable) the composition and to produce cured articles by the use thereof in a molding operation.
Accordingly, it is desired to provide curing bladders with decreased adhesion to cured tires.